Infrared (IR) and UV-VIS absorption have been used for decades for the sensitive detection of molecular species. However, routine detection of absorbances on the order of 10.sup.-5 has only recently been demonstrated, primarily in the area of infrared absorption, and that the detection of absorbances of less than 10.sup.-7 is possible. Present devices utilize laser diodes in both the near and mid-IR and have reached these detected absorbances through frequency modulation, which moves the detection bandwidth to higher frequencies, thereby reducing 1/f (flicker) noise and providing nearly quantum-limited detection. Unfortunately, these devices have limited tunability and lack the multiplex (Fellgett) advantage.
Fourier transform spectroscopy is generally performed using a Michelson interferometer, where the light from a broadband source is divided into equal-intensity beams, with one of the beams traversing a fixed-distance path, and the other traversing a variable-distance path. The beams are then recombined and focused onto a detector. Such systems have the Jacquinot (throughput) and Fellgett (multiplex) advantages. Fourier transform spectroscopy also obtains the Connes advantage; use of a HeNe laser for monitoring the motion of the mirrors (counting fringes) provides an internal calibration source, the stray light advantage; modulation of the individual frequency components eliminates stray light contributions, and finally, no emission contributions; location of the sample after beam recombination and before the detector provides contributions from emission that are DC (no modulation) and these are not observed in the transform plane.
Four major disadvantages of conventional FT spectrometers remain, however: (1) The need for large dynamic range amplifiers and large-bit A/D boards (16-20 bits). These requirements arise from the fact that at zero path difference, the signal is maximized and strong, whereas at large displacements of the movable mirror, the signal is minimal and extremely weak. (2) The use of a moving mirror which causes alignment problems if the instrument is jarred; the instrument is not easily deployed in the field. (3) The inability to directly ratio the sample and background signals simultaneously; a background must be run with the sample removed and then used over time. (4) Routine detectable absorbances are only between 10.sup.-3 and 10.sup.-4.
During the last decade, Hammaker et al. ("Hadamard Transform Raman Spectrometry" in Modern Techniques in Raman Spectroscopy, Edited by J. J Laserna, John Wiley and Sons, Ltd. (1996), pages 143-225) have demonstrated the application of Hadamard Transform (HT) spectroscopy as one method for obtaining increased S/N ratios for fixed integration times. More recently, Radek Sobczynski et al. in "Diode Arrays May Light Up Compact Spectrometers," Laser Focus World, March 1995, pages 75-81, have demonstrated a FT-near-IR instrument which encodes the spectral elements by varying the output of light-emitting diodes (LEDs) over time and observing the carrier amplitudes in the Fourier spectral domain, again providing significant gains in signal-to-noise ratio (S/N) over conventional scanning dispersive instruments. Individual wavelengths are modulated by modulating individual light emitting diodes, each at a different frequency, a plurality of diodes comprising the radiation source. The intensity of each spectral channel can be recovered with a standard Fourier transform of the detected time-domain signal.
Accordingly, it is an object of the present invention to provide an apparatus and method for performing Fourier transform spectroscopy using fixed dispersive optical elements and a polychromatic light source with no moving parts.
Another object of the invention is to provide a Fourier transform spectrometer having increased S/N ratios over scanning dispersive instruments and substantial freedom from 1/f noise.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.